diff --git a/util.c b/util.c
index 5ce2396..4c37ab0 100644
--- a/util.c
+++ b/util.c
@@ -1,6 +1,7 @@
 /* See LICENSE file for copyright and license details. */
 #include <errno.h>
 #include <stdarg.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -93,7 +94,7 @@ bprintf(const char *fmt, ...)
 }
 
 const char *
-fmt_human(size_t num, int base)
+fmt_human(uint64_t num, int base)
 {
 	double scaled;
 	size_t i, prefixlen;
diff --git a/util.h b/util.h
index b474f88..af12962 100644
--- a/util.h
+++ b/util.h
@@ -1,4 +1,6 @@
 /* See LICENSE file for copyright and license details. */
+#include <stdint.h>
+
 extern char buf[1024];
 
 #define LEN(x) (sizeof (x) / sizeof *(x))
@@ -10,5 +12,5 @@ void die(const char *, ...);
 
 int esnprintf(char *str, size_t size, const char *fmt, ...);
 const char *bprintf(const char *fmt, ...);
-const char *fmt_human(size_t num, int base);
+const char *fmt_human(uint64_t num, int base);
 int pscanf(const char *path, const char *fmt, ...);